This invention relates to a method of improving the quality and properties of triniobium tin superconductor during manufacturing operations by controlling the iron content in molten tin or tin alloy dip process. More specifically, this invention relates to identifying a critical amount of iron in a molten tin dip where the iron may influence the subsequent superconducting properties of triniobium tin superconductor.
The intermetallic compound triniobium tin, Nb3Sn, is a type-II metallic superconductor of interest because it has high values of superconducting critical current density in high magnetic fields. In order to achieve high critical current density, the process chosen to form the triniobium tin super-conductor is important. One process currently used is a liquid-solid phase diffusion method. This occurs by diffusion between a solid niobium phase and a liquid tin phase.
In manufacturing environments a technique developed for forming triniobium tin by liquid-solid diffusion is to react liquid tin with solid niobium. An integral step in this method is coating a niobium-based alloy foil with a tin or tin-copper alloy by hot dipping.
Prior to tin dipping, a niobium-based foil is cleaned by mechanical means or chemicals, such as acids. After the foil is cleaned, oxygen is added to the foil by anodizing it with a decomposition anneal or by annealing in an oxygen-rich atmosphere. The decomposition anneal or oxygen-rich anneal is then followed by a hot tin dipping process.
Tin hot dipping proceeds by drawing niobium foil through a molten tin bath. The tin bath is positioned in a chamber containing an inert atmosphere, such as argon. A roller located in the bath transfers the foil through the molten tin, which is held at a temperature greater than 950xc2x0 C. At this temperature, wetting of the tin to the niobium foil is promoted by either the short hot zone above the tin bath or in the tin bath itself. As the foil leaves the bath, a layer of tin coats the surface of the niobium foil. The coated foil then exits through an inert atmosphere which serves to cool and solidify the tin layer.
In a subsequent reaction anneal, the triniobium tin superconductor is formed by the reaction of tin with the niobium. To optimize production throughout and superconducting properties, it is desirable to maximize the rate at which the triniobium tin layer is formed during the reaction anneal. The reaction rate must be consistent if a uniform triniobium tin layer thickness is to be achieved.
So that a sufficient amount of tin for the reaction anneal step is provided, a minimum limit is imposed on the thickness of the tin coating provided by the tin dip. Generally, the thickness of the tin layer is about two to thirty micrometers thick. The thickness of the tin coat influences the final thickness of the triniobium tin layer. This in turn influences the reaction kinetics and critical current density during the formation of the triniobium tin superconductor.
Recently, efforts have been made to improve the critical current density and the critical current of superconducting triniobium tin by ternary and quaternary additions to the tin dip. Improvements in critical current density of triniobium tin have been found by adding gallium, indium, silver, and aluminum to the tin bath. This is the subject of U.S. Pat. No. 4,323,402 to Tachikawa. It has been found that the addition of certain metals to the tin bath may improve the quality of the triniobium tin superconductor.
However, contaminant metals from production sources present in the manufacturing environment or contaminants contained in the tin alloy grade used in manufacturing, may decrease the triniobium tin superconducting properties. This invention has identified iron, present in manufacturing operations and tin alloys, as a contaminant that causes a limiting effect on reaction kinetics and the critical current of triniobium tin, which can be detrimental. Thus, a tolerable amount of iron in the tin bath needs to be determined in manufacturing operations.
It has been discovered that iron contamination of the molten tin-dip alloy in manufacturing operations decreases the rate of formation of triniobium tin superconductor during the reaction anneal process. It has further been discovered that a tin alloy dip having a controlled concentration of iron, one hundred twenty-five parts per million (125 ppm) by weight or less, must be used to ensure the triniobium tin has a reproducible superconducting layer thickness.
In the method of this invention, it is contemplated that the tin alloy dip can comprise a tin-copper alloy dip or an essentially pure tin dip. The tin-copper alloy contains up to about twenty weight percent copper, about one hundred twenty-five parts per million (125 ppm) by weight or less of iron, and the balance substantially tin. The essentially pure tin dip contains about one hundred twenty-five parts per million (125 ppm) by weight or less of iron and about 99.999 percent by weight tin.
Accordingly, this invention comprises a method of manufacturing triniobium tin with an optimized reaction layer that utilizes a tin alloy dip in which the iron concentration is limited to no more than about one hundred twenty-five parts per million. The method of this invention to form triniobium tin superconductor comprises passing an internally oxidized niobium-base substrate through a molten tin alloy dip containing less than or equal to one hundred twenty-five parts per million by weight iron to coat the substrate with a sufficient amount of a tin alloy coating to form the triniobium tin superconductor, and then reaction annealing the substrate with the tin alloy coating at about 900-1200xc2x0 C. in an inert atmosphere for a time sufficient to form the triniobium tin superconductor.
By controlling the amount of iron contamination of the tin dip, the critical current density of the triniobium tin superconductor is increased, as is the reaction kinetics of the reaction anneal process.